Heavy metal fluoride glasses, such as fluorozirconate and fluorohafnate glasses have chemical and physical characteristics which make them ideally suited for a wide variety of optic applications. Multicomponent fluoride glasses, a relatively new class of materials, were first reported by French researchers, M. Chanthanasinh in 1976 and Poulain et al in 1977. These early glasses exhibited strong and very broad IR absorptions near 3 microns due to OH.sup.- contamination and were of poor optical quality due to density gradients, haziness, and precipitated material.
Two problems have plagued the use of fluoride glasses in optic applications, light absorption and light diffraction, both of which relate directly to light transmission. The problem of light absorption is tied to the presence of cation and anion impurities. Many purification methods have attempted to alleviate this problem.
One such method is disclosed by Mitachi and Miyashita, in Elect. Lett., 18, 170 (1982). A "build-in-casting" method is disclosed wherein the cladding glass melt is cast into a cylindrical brass mold, which has been preheated at about glass transition temperature. The melt in the central part of the mold was allowed to flow out, and the core glass melt was instantly cast into the central cylindrical hollow part and annealed. The ZrF.sub.4 :BaF.sub.2 :GdF.sub.3 :AlF.sub.3 (ZBGA) fluoride glass produced by Mitachi and Miyashita demonstrated a measured loss at IR wavelengths of only 21 dB/km at 2.55 .mu.m. Their IR transmission loss spectrum and their estimated impurity concentration data (pg. 170 of the above article), indicate that the measured losses at IR wavelengths are caused by the residual impurities present in the glass, particularly, the cations Fe.sup.2+, Cr.sup.3+, Ni.sup.2+ and the anion, OH.sup.-. (refer to FIG. 1 of the instant application). Please note that this reference is in error in that the description and caption associated with FIG. 2, in fact describes FIG. 1 of said reference (as reproduced in FIG. 1 of the instant application). Therefore, the transition ion content, namely Fe.sup.+2, Cu.sup.+2, Ni.sup.+2, and Cr.sup.+3, must be in the parts per billion range and the anion impurity levels, namely OH.sup.- and O.sup.-2, must also be correspondingly low, in order to achieve the desired IR transmission characteristics.
Many other purification processes have emerged in order to combat the inherent lack of purity which is common to all fluoride materials. Generally, the approach to obtaining pure optic fibers was to purify the starting materials. Reactive atmosphere processing was one method utilized to purify metal fluorides. Examples of this type of processing are illustrated in U.S. Pat. Nos. 3,826,817, 4,519,986 and 4,341,873, all of which are assigned to the present assignee. Processes such as these were very effective in reducing the anion impurity concentration of commercially available zirconium and hafnium tetrafluorides. Removal of the cation impurities present in commercially available zirconium and hafnium tetrafluorides, however, proved exceedingly more difficult.
Typically, sublimation and distillation techniques were effective in removing most of the cation impurities found in commercially received material. Techniques such as sublimation and/or distillation, however, have only been partially effective in removing Fe impurities, due to the relatively high vapor pressure of Fe.sup.+3 (as FeF.sub.3) in the matrix of heavy metal fluorides. These separation techniques are satisfactory for removing most of the cation impurities found in commercially received material, such as the alkaline earth and 4f impurities. However, sublimation and/or distillation have been only partially effective in removing Fe impurities due to the relatively high vapor pressure of Fe.sup.+3 as FeF.sub.3 in the fluoride matrix.
During sublimation or distillation of the zirconium or hafnium tetrafluoride, the concentration of impurities increases in the remaining solid or liquid material so that vaporization of the Fe.sup.+3 begins to occur along with the zirconium or hafnium tetrafluoride to thereby contaminate the sublimate. In order to prevent iron from contaminating the sublimate or distillate, it is necessary to stop the sublimation or distillation after only part of the material has been vaporized. This is undesirable, of course, since the residue must be discarded even though a high percentage of the metal tetrafluoride remains in the residue. As a result, a substantial waste of the material occurs. Further, the degree of purity obtainable by sublimation or distillation alone, does not reduce the Fe content into the ppb range, ie., below 1 part per million (ppm).
Accordingly, U.S. Pat. No. 4,578,252 discloses a method of purifying metal tetrafluorides to produce ultra pure metal tetrafluorides having iron impurity concentrations of below 500 ppb. This method combines electromotive series displacement (ESD) with direct melt electrolysis (DME). More specifically, an electromotive force is applied to the metal tetrafluoride during and preferably prior to distillation or sublimation. The electromotive force is applied to the metal tetrafluoride to electrically plate out relatively volatile iron (Fe) cation impurities. Either of the two processes (ESD or DME) can combine synergistically with distillation or sublimation to produce a distillate or sublimate which has a much lower Fe cation content than that achievable of either sublimation and/or distillation alone. Electromotive series displacement (ESD) and the direct melt electrolysis (DME) method appear to be the best means available for reducing the transition ion impurities (Fe.sup.+2, Cu.sup.+2, Ni.sup.+2, Cr.sup.+3, etc.) and anion impurities (OH.sup.- and O.sup.-2) of fluoride glass, i.e. 1-500 ppb, however, even this method has not completely eliminated these impurities from the fluoride glass material. Because the purification method is applied separately to each component, the impurity degradation results from their mixing, fusion, and quenching to form the fluoride glass. At present, there are no containers, which can suitably interface with the fluoride melt and that are themselves pure at the desired ppb level. Hence, leaching is the problem which accounts for some residual impurities, not the purification method.
Another problem associated with fluoride glass materials is light transmission. In order to effectuate a high level of efficient light transmission through fluoride glass materials, three problems must be solved. Firstly, the absorption of light by the fluoride glass material must be as low as possible. The property of light absorption is controlled by the impurity levels present. Secondly, the level of scattering, caused by small particles or voids in the medium (glass), must also be low. This problem can be solved by carefully choosing the amounts of the requisite components that make up the fluoride glass composition and insuring that the fluoride glass preform is a homogeneous medium. Lastly, diffraction of the light beam outside of its intended path must be minimized.
Fluoride glass is of great importance because of its potential use as a fiber optic material. Obviously, light transmission, particularly diffraction, is a critical factor in enabling the fluoride glass material to operate more effectively. A certain fraction of the light beam as it enters an optic fiber would generally be diffracted outside of the fiber. This would clearly decrease the light transmission of the fiber and decrease the fiber's effectiveness in optic applications. A fiber having greater light transmission efficiency would enable light to be transmitted over greater distances between terminals where it could be amplified before its intensity is degraded to a level where the information it carried became garbled by noise.